Improvements in the performance of television cameras in recent years and reduction in the costs of television cameras make it easy to recapture an image or video image displayed on a screen, such as a display's screen, with a sufficient image quality. However, such technical improvements cause unauthorized use of work, such as a recaptured image or video image. For example, there have been reported cases where a video image screened in a movie theater was recaptured directly with a digital camera, and was copied to a medium, such as a DVD (Digital Versatile Disk), and the medium was on sale illegally.
Nonpatent reference 1 discloses an electronic watermark technology which is intended for prevention of unauthorized use of such a recaptured image. According to this prior art technology, brightness variations with respect to a time direction (e.g., a direction of frames or fields) are provided, as an electronic watermark, to a moving image into which the electronic watermark is to be embedded.
By thus embedding copyright information, as an electronic watermark, into the target image, the user can extract the above-mentioned copyright information even from a recaptured image which is illegally copied from the target image, and can assert the copyright. This translates into prevention of illegal copy of the target image using image recapturing.
Nonpatent reference 1: J. Haitsma and T. Kalker, “A watermarking scheme for digital cinema”, ICIP'01—IEEE, pp. 487-489
In accordance with the prior art electronic watermark technology for electronically watermarking recaptured images which is disclosed by nonpatent reference 1, an electronic watermark which is effective even for recaptured images is embedded into a moving image by changing the brightness of the moving image with respect to a time direction (e.g., a direction of frames or fields). A problem with the prior art electronic watermark technology is, however, that depending on the target moving image into which the electronic watermark is embedded, a difference in the brightness between frame images, which is caused by the brightness variations with respect to the time direction which are provided for the target moving image, can be viewed as a light or dark portion while the moving image is played back.
Furthermore, as a method of increasing the amount of embedding of an electronic watermark, there can be provided a method of dividing each frame image (or each field image) of the target moving image into a plurality of regions spatially, and producing a variation in the pixel values in each of the plurality of divided regions of each frame image according to an embedded bit. A problem with the related art method is, however, that the image quality of the target moving image degrades depending upon the target moving image into which the electronic watermark is to be embedded.
For example, when each frame image or the like is spatially divided into a plurality of regions and brightness modulation is performed on each of the plurality of divided regions, a difference in the brightness occurs among the plurality of divided regions of each frame image due to the brightness modulation and a boundary line between any two of the plurality of divided regions can be viewed while the moving image is played back, depending upon the target moving image into which the electronic watermark is to be embedded.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an electronic watermark embedding method of being able to embed an electronic watermark which is effective for recaptured images into a target image without degrading the image quality of the target image, and an electronic watermark detecting method of being able to detect the electronic watermark with a high degree of precision.
It is a further object of the present invention to provide an electronic watermark embedding apparatus which uses the above-mentioned electronic watermark embedding method, an electronic watermark detecting apparatus which uses the above-mentioned electronic watermark detecting method, a program which makes a computer function as the electronic watermark embedding apparatus, and a program which makes a computer function as the electronic watermark detecting apparatus.